A sheet transport apparatus is known that uses an ultrasonic double feed detection method to detect the feeding of multiple sheets. In the ultrasonic double feed detection method, a threshold value common to all types of sheets is computed in advance and, when starting a document feed unit, this threshold value is automatically set in a detection pulse-height level setting circuit contained in a control unit and is used to detect whether the number of sheets fed out of the document feed unit is more than one.
Related art is disclosed in Japanese Laid-open Patent Publication No. 5-193786.